Structure of Image Sensor
As the applications of the optoelectonic device become more and more popular, the demands for the image sensing device increase rapidly. In general, typical image sensors can be categorized into two main parts, which are the charge coupled device (CCD) and the complementary metal-oxide semiconductor (CMOS).
An image sensor is used for recording a change of a photo signal formed by an image and converting the photo signal into an electronic signal. After recording and processing the electronic signal, a digital image is generated for further outputting or recording. In general, the image sensor is formed by a plurality of photo sensing devices, which are either CCD elements or COMS elements.
A CCD image sensor is formed by a capacitor array having a plurality of metal oxide semiconductor (MOS) elements arranged densely. The manufacturing of the CCD is to deposit a silicon oxide layer on a N type (or P type) single crystal silicon substrate. Then, a PN type MOS capacitor receiving a photo signal is formed on the silicon oxide layer. The MOS capacitor is used for converting the photo signal into an electronic signal. Moreover, the dielectric layer and the signal transmitting circuit are arranged in the boundary of the MOS capacitor array and then integrated into the CCD elements on the single crystal substrate with the powering device. Thus, a CCD image sensor is accomplished.
On the other hand, the CMOS imager sensor is a semiconductor for recording the change of the photo signals. The CMOS mainly includes silicon (Si) and germanium (Ge), so that the N type and P type semiconductors can exist in the CMOS element. Therefore, the currents can be generated by these two complementary semiconductors. After processing and recording such currents, a digital image can be outputted or recorded. The difference between the CCD and CMOS is that the CCD element is formed on the single crystal semiconductor substrate while the CMOS element is formed on the metal oxide semiconductor substrate. However, the working principle of CCD and COMS are identical.
In addition to the abovementioned semiconductor photo sensing device included in the image sensor for converting the photo signals into electronic signals, a plurality of color filters are also included in the image sensor in order to output the color image. Typically, the color filter array included in the image sensor can be either the RGB color filter array or YMC color filter array.
In the conventional technology, the color filter array is disposed above the semiconductor photo sensing device. Furthermore, a micro lens array, such as a convex array, is disposed above the color filter array for converging or condensing the incident light. With the aid of the convex array, the incident light can be condensed to a smaller beam and projected to the specific area of the semiconductor photo sensing device, so as to increase the photosensitivity of the image sensor. Therefore, the typical arrangement of the image sensor, which is formed either by CCD or CMOS semiconductor photo sensing device, mainly includes a lens array, a color filter array and a semiconductor photo sensing device array arranged in sequence along a direction of the incident light.
Please refer to FIG. 1, which shows an arrangement of the conventional CMOS photo sensing element. As can be seen from FIG. 1, the CMOS photo sensing element 10 includes a substrate 11, a first photodiode 12a, a second photodiode 12b, a third photodiode 12c, a metal opaque layer 13, a first color filter layer 14a, a second color filter layer 14b, a third color filter layer 14c, a micro lens layer 15 and a light beam 16. Typically, the first, second and third color filter layers are used for filtering the green, red and blue light respectively.
No matter it is a CCD or a COMS image sensor, a plurality of photo sensing elements, which is also called pixels, are included therein for constructing a hundred thousand level or a million level image sensor. For a CCD image sensor, the electronic photo signals generated in every pixel of each column is transmitted to a buffer in sequence, and then outputted to an AC/DC (ADC) converter disposed near CCD photo sensing elements for amplifying and digitizing the analog electronic signals. The amplified and digitized signals are then transmitted to a processing chip. However, for a COMS image sensor, each pixels is collocated with an ADC converter, so as to amplify and digitize the electronic signal generated by each COMS pixel directly. Therefore, the main differences between the CMOS image sensor and the CCD image sensor are the disposition and the number of the ADC converter.
Please refer to FIG. 2, which shows the pixels layout of a CMOS image sensor, which includes a CMOS photo sensing element and an ADC converter in each pixel. As can be seen from FIG. 2, the CMOS image sensor 20 includes a plurality of pixels 21, each of which has a CMOS image sensor 22 and an ADC converter 23. The feature of the COMS image sensor is that each CMOS image sensor 22 is collocated with an ADC converter 22, so that the electronic signal generated by each COMS pixel can be amplified and digitized directly, and then transmitted to a processing chip for digital signals processing.